Recently, portable electronic equipment, such as cameras, camcorders, portable CD players, portable radio/cassette players, notebook computers, pagers and cellular phones, are widely used. To meet the power needs of such devices, a battery having a higher capacity and a longer life span is needed, and it is economically desirable to reduce the production cost of the battery.
In general, a battery is a device that converts chemical energy into electrical energy by using the contact potential difference between suitable materials, and many kinds of batteries are now available. Batteries are technologically classified as primary batteries, secondary batteries, fuel batteries, or solar batteries. A primary battery, such as a Mn battery, Alkaline battery, Hg battery or oxidized Ag battery, is disposable and cannot be recharged after use. In contrast, a secondary battery is rechargeable after use and can be used repeatedly. Examples of a secondary battery are a lead storage battery, a low voltage Ni/MH battery (metal hydride is used as cathode active material), a sealed type Ni-Cd battery and a Li-based secondary battery, such as a Li metal battery, lithium ion battery (LIB), or lithium polymer battery (LPB). A primary battery has a low capacity and short life span, and may cause environmental pollution because it is not reusable. On the other hand, a secondary battery has several advantages over a primary battery. First, a secondary battery has a longer life span and produces less waste, thereby causing less environmental pollution. Second, a secondary battery has better performance and efficiency than a primary battery because the average voltage of a secondary battery is significantly greater than that of a primary battery. A fuel battery converts heat of combusting into electrical energy, and a solar battery converts light energy into electrical energy.
Materials that are currently used or can be used as the cathode material of a Li-based secondary battery include transition metal oxides such as LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2 O.sub.4, and oxides such as LiM.sub.x Co.sub.1-x O.sub.2 (M.dbd.Ni, Co, Fe, Mn, and Cr). Research for these Li secondary batteries has been vigorously pursued. Recently, a Li ion secondary battery manufactured by Sony Energytec company and Moli Energy Company uses a carbon material as an anode active component, and uses LiCoO.sub.2 and LiMn.sub.2 O.sub.4 as a cathode active component. They used PC (propylene carbonate), EC (ethylene carbonate), DEC (diethyl carbonate), DMC (dimethyl carbonate), LiBF.sub.4 and/or LiPF.sub.6 as an electrolyte.
However, the capacities of the currently used cathode active materials, such as LiNiO.sub.2 or LiCoO.sub.2, are relatively low (100.about.200 mAh/g) compared to theoretical capacity (372 mAh/g) of the anode active material (Li.sub.1 C.sub.6), and only 40-50% of the theoretical capacity is available with the current technology level. Therefore, it is desireable to improve the prior cathode active material so that the theoretical capacity of the cathode can be fully used and to prepare a new cathode active material that has a higher capacity.
In addition, since the cobalt in LiCoO.sub.2 that is widely used as a cathode active material in a Li secondary battery is twice as expensive as nickel and four times as expensive as manganese and detrimental to the health, an alternative to cobalt is required.
Recently, the use of LiMn.sub.2 O.sub.4 as a cathode active material of a Li secondary battery has been researched, especially a process for preparing LiMn.sub.2 O.sub.4, since LiMn.sub.2 O.sub.4 has advantages in cost, charge and discharge characteristics, stability of electrolyte and excellent reversibility.
The cathode active material LiMn.sub.2 O.sub.4 is conventionally prepared by the solid phase reaction of Li.sub.2 CO.sub.3 and MnO.sub.2. This method includes the steps of ball milling the material powder, sintering the powder for 24 hours at a temperature of 700 to 800.degree. C., and repeating the process of ball milling and sintering the powder 2 to 3 times. The active material prepared by the solid phase reaction method shows continuous stability during charge-discharge; however, a large quantity of contaminants are mixed into the active material in the ball milling stage of the raw materials, thereby making it difficult to obtain a homogeneous phase. In addition, since non homogeneous reactions occur more easily, it is difficult to control the size of the powder. Furthermore, high temperature and a long process time are needed for the method. Moreover, the initial capacity of the battery having the LiMn.sub.2 O.sub.4 prepared in this way is low.
According to J. Electrochem. Soc. (Vol. 141, No. 6, pp 1421-1431) and U.S. Pat. No. 5,425,932, LiMn.sub.2 O.sub.4 was prepared by sintering LiCO.sub.3 or LiNO.sub.3 and MnO.sub.3 during 24 hours at a temperature of 800.degree. C., annealing, balling the sintered material and repeating this cycle twice. The LiMn.sub.2 O.sub.4 prepared by this method shows an initial capacity of about 110 mAh/g while charging-discharging the battery at 3 to 4.5V.
In U.S. Pat. No. 5,135,732 by Barboux et al., LiMnO.sub.4 is prepared by the sol-gel method. In this method, a spinel powder of LiMnO.sub.4 was prepared by reacting the lithium hydroxide and manganese and annealing the product at a temperature of 200 to 600.degree. C. In the reaction, NH.sub.4 OH is used as a precipitation agent.
The disadvantage of this method is that the initial capacity is lowered because powder is prepared at low temperature. Also, after adding ammonium hydroxide, the powder deposited at pH 7 is very unstable and easily decomposed during the drying process. Thus, an inert gas atmosphere is absolutely needed in the drying process.